A CCD device (Charge Coupled Device) can be defined as being a semiconductor device wherein, due to the movement of electric charges, storage and collection of charges is possible. These charge transfer devices are used in dynamic, variable storage elements, which characteristically have a high information density. In CCD sensors intended for X-ray imaging, the charges used for image generation, which are formed on physical pixels, can be combined, binned. Binning in a CCD sensor produces virtually larger image pixels. However, in certain situations, handling the charge of these combined pixels may be problematic. Especially at higher signal levels, the signal coming from the image area may be so large that, with the selected binning, the charges can not be combined within the CCD sensor without a risk of saturation of the output amplifier. This can be partially taken into account by designing the read-out register and the charge well of the output amplifier so that they have charge capacities higher than the charge capacity of the pixels. However, if the charge well of the output amplifier is enlarged too much, then the voltage generated on it is reduced, and thus the signal to be produced is also reduced.
At present, the resolution of digital imaging already approaches the level of film-based systems and may in some cases even exceed it. However, it is known that the dynamic range of CCD sensors, the ratio of the maximum signal to the open-circuit noise, to the basic sensitivity, is smaller than the dynamic range of traditional film-based systems. In this context, sensitivity may refer to resolution within background noise, in other words, to the signal size that can be resolved from background noise.
A typical non-cooled CCD device working in MPP mode has a dynamic range of about 10.000:1 . . . 20.000:1. The figure giving the dynamic range represents the ratio of saturation voltage to RMS noise. If this dynamic range is effectively utilized, then it will be possible to exploit as many greyness levels as the A/D conversion used allows. For example, in the case of 14 bits, the total number of greyness levels available will be 16.384.
However, these numeric values are not mutually comparable when film-based and digital systems are compared to each other. Even if a film system would utilize only a small proportion of the total dynamics, in practice there would still be a very large number of greyness levels available. If one considers e.g. one thousandth (1:1000) of the dynamics of the film, it can be shown that this range is divided into more than 16 separate levels.
Moreover, CCD based sensors do not forgive in situations of overexposure as do film-based systems, in which the reciprocal law (gradual saturation, s-curve of the film) causes a soft saturation and along with this a “compression”, i.e. an expansion of dynamics. It is thus conceivable that one is dealing here with an in-built non-linearity, which can also be understood as a kind of gamma correction in the film itself. In digital sensors, a visible boundary, artefact, is produced as soon as the maximum point of the dynamic range is reached.
The signal amplifier and A/D converter (A/D, Analog-to-Digital) connected after the sensor are designed with an aim to enable the entire dynamic range produced by the CCD sensor to be utilized. As mentioned above, existing CCD sensors can produce a dynamic range even exceeding 20000:1. In an ideal situation, the quantization step of the A/D converter is slightly below the CCD sensor's own noise level. However, this would mean that, in order to digitize the image, it would be necessary to use fast A/D conversion exceeding 14 bits.
In other words, the charge received and contained in a pixel may be so large that it can not be handled in a binning situation in the read-out register and/or in the charge well of the output amplifier without a risk of saturation. Especially in the case of larger binning operations, for example 3×3 and 4×4 (horizontal direction×vertical direction, the number of times of reading from the image area to the output register and from the output register to the read-out well), this becomes a problem. The image formed in the image area is not saturated and it is perfectly usable, but it can not be read out without saturation with the binning in question.
Both in cephalostatic use (Ceph) and in the panoramic image (Pan), there are areas below the jaws, which receive direct radiation without any intermediate tissue attenuating the radiation. In these situations, among others, there is an obvious risk of saturation if the system is otherwise tuned for optimal reception of the signal coming through the object. This makes imaging of e.g. soft tissue areas impossible, and in the areas of saturation all the image information is lost.